The invention relates to a swash plate pump, particularly a swash plate pump used as a helm pump for providing pressurized fluid to a hydraulic fluid actuator which controls a marine vessel tiller.
Swash plate pumps have been used for many years as helm pumps to supply pressurized fluid to hydraulic actuator cylinders to actuate the tiller. Such pumps have a housing and a pump rotor journalled for rotation within the housing by rotating a steering wheel. The rotor has a plurality of circumferentially disposed cylinders containing respective pistons which are held resiliently against an inclined swash plate as the rotor is rotated relative to the housing. Such pumps are usually a portion of a closed hydraulic circuit in which fluid from the pump is supplied to one side of a piston of the actuator cylinder, and displaced fluid from an opposite side of the piston is returned to the housing to be fed back into the rotor. In effect the housing serves as a sump to receive fluid returned from the actuator cylinder.
To provide an operator with a variable "gear ratio" for actuation of the tiller, some helm pumps have a variable displacement by providing a swash plate with a variable angle. In this way, in heavy seas the swash plate is set almost perpendicularly to a pump axis and the helm pump delivers a relatively small volume of fluid for a given number of turns of the wheel. However, in lighter seas, the swash plate is set more obliquely and the helm pump delivers a greater volume of fluid for the same number of turns of the wheel. One helm pump of this general type is disclosed in U.S. Pat. No. 3,935,796 (Wood), which shows screw thread means for adjusting angle of inclination of the swash plate. Other U.S. patents of this general type of pump include U.S. Pat. Nos. 3,384,028 (Thoma), 3,190,232 (Budzich) and 2,769,393 (Cordillo et al).
In some variable displacement pumps, outer ends of the pistons contacting the swash plate are provided with a partially spherical contact point which can accommodate different angles of inclination of the piston to the swash plate. While a variable displacement pump is desirable in some circumstances, because the ends of the pistons must accommodate different angles of the swash plate, a relatively small radius contact tip is commonly used, which is subject to relatively high rates of wear due to relatively high bearing stresses as the pistons sweep the swash plate. To reduce wear rates, it is common to insert a steel ball at an outer end of the piston located on a central axis of the cylinder, which, while reducing wear problems, increases manufacturing costs.
In addition, whenever a hydraulic system is installed or serviced, air trapped in the system must be purged, usually by displacing air and fluid through bleed valves or purge valves. When a helm pump is driven directly by a vertically disposed wheel at the helm, an axis of rotation of the rotor is generally horizontal and it is usually necessary to provide a bleed valve for each cylinder. In instances where a steel ball is used to reduce wear of the piston, usually two or three arcuate portions of bleed conduits are provided on opposite sides of the ball to permit purging of the cylinders, irrespective of the direction of rotation of the cylinders. Such conduits are located away from the central axis of the cylinder where the conduits break out of the piston, as the central axial position is occupied by the steel ball. In general, it is a relatively costly manufacturing procedure to install a steel ball at an end of a piston and provide the several arcuate bleed conduits extending around the ball.